1. Field of the Invention
The present invention relates to a device (hereinafter referred to as a light emitting device) including an element (hereinafter referred to as a light emitting element) having a luminous material interposed between electrodes. Particularly, the present invention relates to a light emitting device including a light emitting element (hereinafter referred to as an EL element) using, as a luminous material, an organic compound in which EL (Electro Luminescence) is obtained. Incidentally, an organic EL display and an organic light emitting diode (OLED: Organic Light Emitting Diode) are included in the light emitting device of the present invention.
The luminous material which can be used for the present invention includes any luminous materials which emit light (phosphorescence and/or fluorescence) through singlet excitation, triplet excitation, or both excitations.
2. Description of the Related Art
In recent years, research of an EL element has proceeded in which a thin film capable of obtaining EL and made of an organic compound is interposed between an anode and a cathode, and development of a light emitting device using self luminescence of the EL element has proceeded. In the development of this light emitting device, although a passive matrix type has been the mainstream, there is a fear that when a pixel portion becomes highly fine, the light emitting brightness of the EL element must be increased, so that the reliability (the long life of the EL element) can not be secured.
Then, recently, attention has been paid to an active matrix type in order to attain a highly fine display. The active matrix type light emitting device has a feature that an input signal is controlled by a semiconductor element provided in each pixel to make an EL element emit light, and a transistor is generally used as the semiconductor element.
A typical pixel structure is such that two transistors are included in a pixel and have different roles respectively, and the light emitting brightness of the EL element can be controlled. As a result, a light emitting period is almost equivalent to one frame period, and even if a pixel portion becomes highly fine, it becomes possible to display an image while the light emitting brightness is suppressed. Thus, it has been considered that the active matrix type is effective for a light emitting device including a highly fine pixel portion.
However, in the active matrix type light emitting device, a plurality of transistors are formed on the same substrate, and it is difficult to ensure the yield as compared with a passive matrix type of a simple structure. Besides, since a manufacturing process of a transistor is relatively complicated, there is a fear that the manufacturing cost becomes high as compared with the passive matrix type light emitting device. Further, in that case, there is a fear that the unit cost of an electric instrument using the active matrix type light emitting device as its display portion is also raised.